U.S. Pat. No. 5,268,708 discloses an image processing apparatus arranged to form an intended image on a receiver secured to the periphery of an imaging drum while the drum is rotated past a printhead. A translation drive then traverses the printhead axially along the imaging drum, in coordinated motion with the rotating imaging drum. A scanning subsystem or write engine provides the scanning function by generating a once per revolution timing signal to data path electronics as a clock signal while the translation drive traverses the printhead axially along the imaging drum in a coordinated motion with the imaging drum rotating past the printhead.
The lathe bed scanning frame provides the structure to support the imaging drum and its rotational drive. The translation drive with the printhead are supported by two translation bearing rods that are substantially straight along their longitudinal axis and are positioned parallel to the imaging drum and a lead screw.
The above mentioned motion is accomplished by means of a DC. servo motor and encoder which rotates the lead screw that is, typically, aligned parallel with the axis of the imaging drum. The printhead is placed on the translation stage member supported by the front and rear translation bearing rods. The translation bearing rods are positioned parallel to the imaging drum, so that it automatically adopts the preferred orientation with respect to the surface of the imaging drum. The translation stage member and printhead are attached to a rotatable lead screw (having a threaded shaft) by a drive nut and coupling. The coupling is arranged to accommodate possible misalignment of the drive nut and lead screw so that only rotational forces and forces parallel to the lead screw are imparted to the translation stage member by the lead screw and drive nut. The lead screw rests between two sides of the lathe bed scanning frame of the lathe bed scanning subsystem or write engine, where it is supported by deep groove radial bearings. At the drive end the lead screw continues through the deep groove radial bearing, through a pair of spring retainers that are separated and loaded by a compression spring to provide axial loading, and to a DC. servo drive motor and encoder. The DC. servo drive motor induces rotation to the lead screw, moving the translation stage member and printhead along the threaded shaft as the lead screw is rotated. The lateral directional movement of the printhead is controlled by switching the direction of rotation of the DC. servo drive motor which, in turn, changes the direction of rotation of the lead screw.
Although the presently known and utilized image processing apparatus is satisfactory, it is not without drawbacks. The motor that drives the lead screw must operate in both directions and is required to start and stop quickly. The lead screw shaft itself is generally straight, but will have some out-of-round tolerances that the motor must be able to accommodate. As a result of these factors, this motor is subject to mechanical stress and its coupling to the lead screw shaft typically requires multiple parts that are assembled to work together to compensate for mechanical tolerance differences. The encoder coupling to the imaging drum also requires multiple components.
Known approaches that have been used to solve the above mentioned problems include complex coupling techniques, with multiple components used to couple the drive motor shaft to the lead screw shaft and spring-loading of the lead screw shaft to provide axial loading against these coupling components and against the drive motor shaft. This approach compensates for possible misalignment of the motor shaft and lead screw and allows sufficient "play" for minor mechanical tolerances. This approach requires alignment of the motor shaft and lead screw, both mounted on the frame. Using the above-mentioned approach, there is some torsional stress on the motor and on motor bearings as the motor starts and stops repeatedly during operation. This effectively reduces motor reliability and shortens motor life.
A similar coupling problem also presents itself for the mounting of the drum encoder. Mechanical tolerances of the shaft for the imaging drum can stress bearings in the encoder. To minimize the effect of tolerance differences, couplings using multiple components are employed for the drum encoder.
A solution that is well-known in the art for reducing torsional stress on a motor is direct attachment of the motor shaft to the lead screw shaft. With this approach, the lead screw then effectively supports the motor so that the motor is not mounted to a frame. Instead, a rotational stop, attached to the motor frame, prevents the motor from turning with the shaft. The same approach can be used for encoders.
Conventional motor stops used for this type of application are spring-held. Spring tension holds the rotational stop firmly against a fixed point, while allowing some amount of free-play of the motor frame as it turns the lead screw. However, springs present some drawbacks, including wear over time and susceptibility to vibration. Designers must carefully specify spring size, type, and strength. Spring retention requires additional hardware on the motor stop and on the equipment frame. Careful handling of springs is required during assembly and repair.